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NMR Characterization of Micro- to Nanoporosity Within Diagenetically Complex Carbonate Reservoirs: Mississippian-Aged Carbonates (Reno County, Kansas)

Abstract

The heterogeneous nature of pores and the resultant complex pore connectivity at multiple scales in carbonates requires that pore characterization workflows integrate the contribution of microporosity. This study characterizes micro- to nano-meter sized pores in a variety of carbonate fabrics and facies by combining nuclear magnetic resonance (NMR) response with multiscale digital image analysis (DIA), and petrographic facies descriptions. NMR transverse relaxation times (T2) provide direct information on porosity, pore sizes, and pore size distribution. Consequently, rock fabric influences the overall geometry of T2 distribution signals. Empirically-derived T2 cutoff values are used in partitioning bound fluids associated with microporosity from movable fluids related to larger pores; thus by comparing T2 cutoff to microporosity estimated from DIA, additional insights into the contribution of microporosity in different facies can be observed.Facies examined in this study include skeletal peloidal packstones and grainstones, spiculitic chert, dolomitized grainstones, and skeletal wackestones to mudstones - all from a core in the diagenetically complex “Mississippian Lime” reservoir. Porosity ranges from 3.6% to 46%, with associated permeability values between 0.02mD and 56mD in these rocks. Micropore types identified from ion-milled surfaces include intercrystalline and separate-vug matrix pores which have varying contributions to effective porosity. DIA-measured pore sizes range from 84nm to 3.08mm, over four orders of magnitude. T2 cutoff values vary from 24ms in dolomitic grainstones to 115ms in the spiculitic cherts, and bound volume estimations increase with percentage micro- to nanoporosity. Consequently, for NMR saturation estimates in carbonate rocks, it is recommended to apply T2 cutoff values that take the multimodal nature of the pore system of carbonate rocks into consideration. Additionally, these observed effects of percentage microporosity, dominant micropore types, and micropore geometry on permeability, fluid saturation, and T2 cutoff are assessed using linear regression models. The approach utilized in this study provides a basis for estimating the effects of microporosity on NMR measurements and thus enhancing current industry standards for interpreting NMR data in carbonate rocks.